Land Grid Array interposers (LGA) are an important class of area-array socket connectors used to reversibly connect a chip module to a printed circuit board (PCB, PWB). They consist of a two-dimensional array of contacts, each of which spans from one side of an insulating carrier plane to the other, is compliant and conducts electricity from one extreme end point on the contact to the other extreme. LGAs are typically held under force sandwiched between a chip module and a printed wiring board. The force is typically provided by an external spring and hardware system.
While offering many advantages, LGAs do suffer from some problems, notably, a wide distribution of forces across the array resulting from non-planar mating surfaces of the module bottom and/or the PCB top. This can lead to unreliable electrical performance. This mating surface nonuniformity may stem from inherent warp in the PCB or chip module, dynamic bending under the applied load, non-uniformity in the applied load, or any combination of these factors.
LGA contacts are made to be compliant in order to accommodate some mating surface nonuniformity. Nonetheless, an unfavorable distribution of button forces is found in practice. This becomes worse as the size of the array becomes larger, with smaller applied forces, and with decreased rigidity and planarity of the module and PCB materials. This is particularly problematic in organic packaging modules of the SLC type which suffer from poor initial planarity, low rigidity, and is limited to the use of low applied force to minimize internal damage to module circuitry.